inits

Chris Kuklewicz wrote:
> Aaron Denney wrote:
>>> On 2006-04-08, Nils Anders Danielsson <nad at cs.chalmers.se> wrote:
>>>>> On Fri, 07 Apr 2006, "Spencer Janssen" <spencerjanssen at gmail.com> wrote:
>>>>>>>>>>> inits xs = [] : (zipWith take [1..] $ map (const xs) xs)
>>>>>>>>> As this version performs much better and will work as a drop in
>>>> replacement, I suggest that it be included in the hierarchical
>>>> libraries.
>>>>>>> It is not a drop in replacement. The original inits is strict, this
>>> one isn't.
>>>>>> The specification of inits (from the Haskell 98 report):
>>>>>> inits :: [a] -> [[a]]
>>> inits [] = [[]]
>>> inits (x:xs) = [[]] ++ map (x:) (inits xs)
>>>>> Is that a property many programs depend on? I'd actually call that a
>> bug of the original.
>>>>>> It may break some things:
>> head (Data.List.inits undefined) is an error
>> head (New.Spiffy.inits undefined) is []
>>Is the head of the inits of undefined really an error?
Since the head of inits [] is also [] ...
But if you really want that undefined to produce an error.. you could just :
inits' xn@(_:_) = zipWith take [0..] $ map (const xn) $ undefined:xn
inits' _ = undefined